Their new study found that hospitalized heart attack patients were about half as likely to have received a flu vaccination and about twice as likely to have gotten sick with the flu as people living in the same city who had not suffered a heart attack, leading them to conclude that flu shots might shield against heart attacks.

Not everyone agrees it's fair to make that claim based on this study's findings, however.

"To draw any definite conclusions from this study would be a bit of a jump at this point," says Dr. Tara Narula, a cardiologist at Lenox Hill Hospital in New York City who was not involved in the research.

Generally, in this kind of a study, researchers try to study two groups of people who are very similar except for their exposure to one or two variables of interest.

But in this study, which was published online Aug. 21 in the journal Heart, the two groups that were compared were very different, making it tougher to tease out the true effects of the flu or flu shots on heart attack risk, Narula said.

Researchers studied more than 550 people. About half of them had recently been hospitalized for a heart attack, while the other half lived in the same city but had no history of heart attacks or recent cardiovascular problems.

Heart attack patients were much younger, on average, than healthy adults in the same city. The majority of heart attack patients were between the ages of 40 and 64, while almost three-quarters of the adults who had not had a heart attack were older than 65. They also were more likely to be male, to be current or former smokers and to report other health problems.

And 33.5 percent of heart attack patients had received a flu shot the year they entered the study compared with nearly 65 percent of patients without a history of heart attack.

Influenza infections were diagnosed in about 12 percent of heart attack patients, compared to slightly less than 7 percent of adults who had never had a heart attack.

After researchers adjusted their data to account for the major differences between study groups, they found that getting the flu wasn't associated with having a heart attack, but flu shots were. Flu shots were associated with a 45 percent reduced risk having a heart attack.

The study was sponsored by GlaxoSmithKline, a company that manufactures several flu vaccines, and study authors reported receiving grant funding from several flu vaccine manufacturers.

"With this type of study, we can't say that giving flu vaccination will prevent a heart attack," Narula said. "It sort of intuitively makes sense that by decreasing infection rates or inflammation, you could, theoretically, prevent heart attack, which is a pro-inflammatory state. That being said, we don't have enough data yet to really prove that one is definitively linked to the other."

Narula noted that previous studies have shown links between infections of all kinds and the worsening of heart problems, and she counsels her patients to do everything they can to stay healthy to protect their hearts.

Flu shots very well may play an important role in preventing infections in this vulnerable population, she said. "It's just not something we've proven yet," she said.

Other experts agree.

"I struggle with the whole notion, in this study, that the flu isn't associated with heart attacks, but vaccination is associated with protection against heart attacks," said cardiologist Dr. Michael Blaha, director of clinical research at the Johns Hopkins Ciccarone Center for the Prevention of Heart Disease, in Baltimore.

"But I don't struggle with the notion -- because I see it in everyday practice -- that people with the flu tend to get heart attacks more often," he said.

Blaha said all kinds of infections, as well as other physical stresses, such as shoveling snow, can trigger heart attacks.

"It does seem reasonable to me that curbing influenza infections may have a positive effect on reducing acute heart attacks," he said.

For that reason, he said, people who know they have heart disease should be especially vigilant about getting an annual flu shot. Everyone else, he said, should continue to get vaccinated to guard against the flu, which can be plenty serious on its own.

The U.S. Centers for Disease Control and Prevention recommends flu shots for everyone six months and older, especially people with certain medical conditions, pregnant women and those aged 65 and older.

More information

For more about flu shots, head to the U.S. Centers for Disease Control and Prevention (http://www.cdc.gov/flu/protect/keyfacts.htm ).

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.